Add symlink setup instructions to README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

slides_template/README

@@ -64,3 +64,7 @@ Example:
 This creates my-talk/ populated with the template files, with the main
 source file renamed to my-talk.tex. Fonts are symlinked from the source
 repository rather than copied, so the repository must remain in place.
+
+Symlink the script into your PATH with:
+
+  ln -s ~/tex/beamer-mako/new_beamer_presentation ~/bin/